kexec-tools: Refine kdump device_tree sort

kexec-tools: Refine kdump device_tree sort

The commit b02d735bf was to rearrange the device-tree entries, and
assumed that these entries are sorted in the ascending order. but
acctually when I was validating kexec and kdump, the order of
serial node still is changed. So the patch is to sort these entries
by the directory name in ascending order.

(From OE-Core rev: c8722b510f779cd20757477a7f7a7a2a35b9a9c5)

Signed-off-by: Yang Wei <Wei.Yang@windriver.com>
Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>